A conventional smart system includes a vehicular device equipped to a vehicle and a portable device carried by a user. The vehicular device transmits an LF-band LF signal of approximately 100 kilohertz to the portable device. The LF signal contains a wake code to instruct activation and a challenge code for authentication. The portable device transmits an RF-band response signal of ten to several tens of megahertz to the vehicular device in response to reception of the LF signal. The response signal is encrypted using the challenge code. The vehicular device verifies whether or not the response signal is transmitted from an authenticated portable device. When the verification result is normal, the smart system provides control to unlock a vehicle door or permits to start a vehicle engine.
In the above-described smart system, as illustrated in FIG. 8, repeaters A and B may be placed near the vehicle and the portable device, respectively. The repeaters A and B may enable the smart system to perform communication between the vehicular device and the portable device. In this configuration, there remains controversy over a relay attack technique that enables to provide control over unlocking the vehicle door or start the vehicle engine even though the vehicle and the portable device are distant from each other.
A technology to solve this issue controls a vehicular device equipped to a vehicle to transmit an LF-band LF signal containing a WAKE UP code (Wake) and a vehicle authentication code. The technology counts a delay time that occurs when receiving a response signal transmitted from a portable device in response to reception of the LF signal. The technology identifies the possibility of a relay attack using a repeater when the delay time exceeds an appropriate range. When the delay time exceeds the appropriate range, the technology inhibits control over unlocking the vehicle door or permission to start the vehicle engine (e.g., see patent literature 1).
As illustrated in FIG. 9, when transmitting the challenge code and the wake code from the vehicular device to the portable device, the apparatus described in patent literature 1 uses LF-band signal same as the wake code and also uses LF-band signal as the challenge code. In the configuration that uses an LF-band signal for the challenge code transmitted from the vehicular device to the portable device, a delay of several tens of microseconds and a clock variation may occur when the portable device demodulates the LF-band challenge code, for example. There is no alternative but to rely on the accuracy comparable to several tens of microseconds in order to count the time period from when the vehicular device transmits the wake code or the challenge code to the portable device to when the vehicular device receives a response signal from the portable device. Thus, the detection of a relay attack based on the challenge code adopting the LF-band signal has a low accuracy.